Ion conductive composite membrane using inorganic conductor and method of manufacturing the same

ABSTRACT

An ion-conductive composite membrane and a method of manufacturing the same, the membrane including phosphate platelets, a silicon compound, and a Keggin-type oxometalate and/or Keggin-type heteropoly acid, wherein the phosphate platelets are three-dimensionally connected to each other via the silicon compound. An electrolyte membrane having an ion-conductive inorganic membrane or an ion-conductive organic/inorganic composite membrane effectively prevents crossover of liquid fuel without the reduction of ion conductivity in a liquid fuel cell, thereby allowing for the production of fuel cells having excellent performance.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No.2005-62927, filed on Jul. 12, 2005, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein in its entiretyby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

An aspect of the present invention relates to an ion-conductivecomposite membrane and a method of manufacturing the same, and moreparticularly, to an ion-conductive composite membrane that effectivelyprevents crossover of liquid fuel without the reduction of ionconductivity in a fuel cell and a method of manufacturing the same.

2. Description of the Related Art

A group of fuel cells form an electrochemical apparatus in whichchemical reaction energy between oxygen and hydrogen contained in ahydrocarbon-based material, such as methanol, ethanol, or natural gas,is directly converted into electrical energy. Since the energyconversion process of fuel cells is very efficient and environmentallyfriendly, various fuel cells have been researched.

Fuel cells can be categorized into phosphoric acid type fuel cells(PAFC), molten carbonate type fuel cells (MCFC), solid oxide type fuelcells (SOFC), polymer electrolyte membrane fuel cells (PEMFC), alkalitype fuel cells (AFC), and the like, according to the electrolyte thatis used. These fuel cells operate based on the same principle, but havedifferent fuels, different operating temperatures, different catalysts,different electrolytes, etc. The PEMFC is effective for use in smallstationary electric power plants and transportation systems because thePEMFC has several advantages such as low temperature operation, highoutput density, rapid start-up, and quick response for the change ofoutput requirement.

The main portion of a PEMFC is a membrane electrode assembly (MEA). AMEA typically includes a polymer electrolyte membrane and two electrodesattached to opposite sides of the polymer electrolyte membrane andrespectively acting as a cathode and anode.

The polymer electrolyte membrane functions as a separation membranepreventing the direct contact of oxidizers and reducing agents, anelectric insulation between the two electrodes, and a proton conductingmedium. Accordingly, the polymer electrolyte membrane is required tohave high proton conductivity, high electric insulation, low reactantpermeability, high thermal, chemical, and mechanical stability inoperating conditions of fuel cells, and low costs.

To satisfy these requirements, various polymer electrolyte membraneshave been developed. Perfluoropolysulfonic acid membranes such as aNAFION membrane are widely used because of their good durability andperformance. However, sufficient moisture should be supplied to properlyoperate the NAFION membrane and the operating temperature should belower than 80° C. to prevent moisture from being lost. In addition,carbon-carbon bonds in a backbone in the NAFION membrane may be attackedby oxygen (O₂) making the bonds unstable in operating conditions of fuelcells.

In the case of DMFCs, a methanol solution is supplied to an anode asfuel, and thus some of the unreacted methanol permeates into a polymerelectrolyte membrane of the DMFC. The permeation of the methanol intothe polymer electrolyte membrane causes the electrolyte membrane toswell and the methanol diffuses to a cathode catalyst layer. This isreferred to as ‘methanol crossover’. Accordingly, since the methanol isdirectly oxidized at a cathode at which electrochemical reduction ofprotons and oxygen occurs, the potential of the cathode decreasessignificantly thereby degrading the performance of the fuel cells.

Such a problem commonly occurs in other fuel cells using liquid fuelcontaining polar organic fuel.

Prevention of the crossover of polar organic liquid fuel such asmethanol or ethanol has been actively researched. For example, inorganicfillers were dispersed in a polymer electrolyte matrix to obtain amembrane (U.S. Pat. Nos. 5,919,583 and 5,849,428) or an inorganiccation-exchange material was mixed with NAFION to form aorganic/inorganic composite membrane (U.S. Pat. No. 6,630,265). For thispurpose, the development of nano composite materials and the formationof an exfoliated layer of an inorganic material in polymers have beenactively researched.

In the development of the nano composite materials, an inorganicmaterial having proton conductivity is obtained from clay and exfoliatedusing polymers, or polymers are intercalated into gaps. This approachhas brought about many improvements, but has not completely preventedthe crossover of the polar organic liquid fuel.

An exfoliated layer of an inorganic material is formed by adding ionconductivity to an exfoliated inorganic material and coating the same ona base material. However, this method cannot completely prevent thecrossover of the polar organic liquid fuel, and essentially requires thebase material.

SUMMARY OF THE INVENTION

An aspect of the present invention provides an ion-conductive inorganicmembrane that effectively prevents crossover of liquid fuel without thereduction of ion conductivity.

Another aspect of the present invention provides an ion-conductiveorganic/inorganic composite membrane that effectively prevents crossoverof liquid fuel without the reduction of ion conductivity and has highmechanical strength.

Another aspect of the present invention provides a method ofmanufacturing the ion-conductive inorganic membrane.

Another aspect of the present invention provides a method ofmanufacturing the ion-conductive organic/inorganic composite membrane.

Another aspect of the present invention provides a membrane electrodeassembly including the ion-conductive inorganic membrane or theion-conductive organic/inorganic composite membrane.

Another aspect of the present invention provides a fuel cell includingthe ion-conductive inorganic membrane or the ion-conductiveorganic/inorganic composite membrane.

According to another aspect of the present invention, there is providedan ion-conductive inorganic membrane including: phosphate platelets; atleast one silicon compound selected from the group consisting ofsilicate, siloxane, and silane; and a Keggin type polyoxometalate or aKeggin-type heteropoly acid, wherein the phosphate platelets arethree-dimensionally connected to each other via the at least one siliconcompound.

According to another aspect of the present invention, there is providedan ion-conductive organic/inorganic composite membrane including: aporous base material; and an ion-conductive inorganic material whichincludes phosphate platelets, at least one silicon compound selectedfrom the group consisting of silicate, siloxane, and silane, and aKeggin-type polyoxometalate or a Keggin-type heteropoly acid, whereinthe phosphate platelets are three-dimensionally connected to each othervia the at least one silicon compound, wherein the ion-conductiveinorganic material is coated on a surface of the porous base material orimpregnated throughout the entire porous base material.

According to another aspect of the present invention, there is provideda method of manufacturing an ion-conductive inorganic membraneincluding: mixing 100 parts by weight of phosphate platelet, 20 through250 parts by weight of at least one silicon compound selected from thegroup consisting of silicate, siloxane, and silane, 0.1 through 10 partsby weight of a Keggin-type polyoxometalate or a Keggin-type heteropolyacid, and 10 through 30 parts by weight of acid to form a gel mixture;applying the gel mixture onto a substrate and drying the gel mixtureapplied on the substrate; and removing the substrate.

According to another aspect of the present invention, there is provideda membrane electrode assembly (MEA) that includes a cathode having acatalyst layer and a diffusion layer, an anode having a catalyst layerand a diffusion layer, and an electrolyte membrane disposed between thecathode and the anode, wherein the electrolyte membrane includes theion-conductive inorganic membrane or the ion-conductiveorganic/inorganic composite membrane.

According to another aspect of the present invention, there is provideda fuel cell that includes a cathode having a catalyst layer and adiffusion layer, an anode having a catalyst layer and a diffusion layer,and an electrolyte membrane disposed between the cathode and the anode,wherein the electrolyte membrane includes the ion-conductive inorganicmembrane or the ion-conductive organic/inorganic composite membrane.

Additional aspects and/or advantages of the invention will be set forthin part in the description which follows and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 is a schematic conceptual diagram of an ion-conductive membraneaccording to an embodiment of the present invention;

FIGS. 2A and 2B are scanning electron spectroscopy (SEM) imagesillustrating porous base materials used in manufacturing ion-conductiveorganic/inorganic composite membranes of Examples 1 through 3 andExamples 4 through 6, respectively, according to an embodiment of thepresent invention;

FIG. 3 is a SEM image illustrating a surface of the ion-conductiveorganic/inorganic composite membrane manufactured in Example 2;

FIGS. 4A and 4B are SEM images illustrating an inner cross-section and asurface of the ion-conductive organic/inorganic composite membranemanufactured in Example 5, respectively; and

FIG. 5 is a graph of ion conductivity of the ion-conductiveorganic/inorganic composite membranes manufactured in Examples 1 through3 and Examples 4 through 6.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The embodiments are described below in order to explain thepresent invention by referring to the figures.

FIG. 1 is a schematic conceptual diagram of an ion-conductive membraneaccording to an embodiment of the present invention. The ion-conductiveinorganic membrane includes: phosphate platelets 10; at least onesilicon compound 20 selected from the group consisting of silicate,siloxane, and silane; and Keggin-type polyoxometalate 30 and/orKeggin-type heteropoly acid 30, wherein the phosphate platelets 10 arethree-dimensionally connected to each other via the at least one siliconcompound 20.

In the ion-conductive inorganic membrane according to an embodiment ofthe present invention illustrated in FIG. 1, the phosphate platelets 10are three-dimensionally connected to each other via the at least onesilicon compound 20, and a plurality of Keggin-type polyoxometalates 30and/or Keggin-type heteropoly acids 30 are dispersed in the structure.

The phosphate platelets 10 may be α-zirconium phosphate, clay, orgraphite oxide, but preferably α-zirconium phosphate. However, thephosphate platelets 10 are not limited to these platelets.

The phosphate platelets 10 are ion conductive and prevent the permeationof liquid molecules such as water or methanol. The phosphate platelets10 can be stacked in order to prevent the permeation of methanol.

However, when only phosphate platelets 10 are employed, liquid such aswater or methanol permeates through the gaps between the phosphateplatelet particles. Thus, the phosphate platelets 10 at most retard thepermeation of methanol and reduce the amounts of methanol, but cannoteffectively prevent methanol crossover.

In the ion-conductive inorganic membrane according to an embodiment ofthe present invention, the phosphate platelets 10 arethree-dimensionally densely connected to each other using the at leastone silicon compound 20 so that methanol molecules and/or watermolecules cannot permeate into the gaps between the phosphate platelets10.

The phosphate platelets 10 are exfoliated and may have thicknesses of0.5 through 10 nm. When the thickness of the phosphate platelets 10 isless than 0.5 nm, the mechanical strength thereof is weak. When thethickness of the phosphate platelets 10 is greater than 10 nm, thedispersion thereof becomes lower so that the prevention of the liquidfuel crossover is reduced.

The at least one silicon compound 20 to connect the phosphate platelets10 is, for example, colloidal silica, polydimethylsiloxane (PDMS),alkoxysilane, chlorosilane, or a combination thereof.

The at least one silicon compound 20 includes silicate and alkoxysilanehaving an organic functional group such as silicate ester. The silicatehaving the organic functional group may be colloidal silica, forexample, LUDOX available from Aldrich Fine Chemicals Co. The at leastone silicon compound 20 is, for example, tetraethylorthosilicate (TEOS),tetramethylorthosilicate (TMOS), 1,2-bis(trimethoxysilyl)ethane,octyltrimethoxysilane, tetraethyl silicate, tetraisopropyl silicate,tetramethyl silicate, tetramethoxysilane, tetraethoxysilane,tetraisopropoxysilane, propyltrimethoxysilane, ethyltrimethoxysilane,methyltrimethoxysilane, methyltriethoxysilane, aminoalkoxysilane,silicontetrachloride (SiCl₄), trichloro(dichloromethyl)silane(Cl₂CHSiCl₃), trichloro(hexyl)silane (CH₃(CH₂)₅SiCl₃),trichloro(isobutyl)silane ((CH₃)₂CHCH₂SiCl₃), trichloro(octadecyl)silane(CH₃(CH₂)₁₇SiCl₃), trichlorooctylsilane, trichloro(phenethyl)silane(C₆H₅CH₂CH₂SiCl₃), trichloro(phenyl)silane, trichloro(propyl)silane,trichloro(methyl)silane, trichloro(chloromethyl)silane,dichloro(dimethyl)silane, isobutyl(trimethoxy)silane, and a combinationthereof, but the present invention is not limited thereto.

The weight ratio of the phosphate platelets 10 to the at least onesilicon compound 20 may be 1:0.2 through 1:2.5. When the amount of theat least one silicon compound 20 is smaller than the minimum of therange, the formation of a network becomes weak so that the prevention ofthe permeation of the liquid fuel and the mechanical strength of themembrane are reduced. When the amount of the at least one siliconcompound 20 is larger than the maximum of the range, the ionconductivity decreases because of the relative reduction of the portionhaving ion conductivity or the phase separation.

Since the phosphate platelets 10 in the present membrane have weaker ionconductivity than other polymer electrolyte membranes, ion-conductivematerials are added. The ion-conductive material is Keggin-typepolyoxometalate 30 and/or Keggin-type heteropoly acid 30.

The polyoxometalate 30 and the heteropoly acid 30 have a negativelycharged polyhedral cage structure, and are electrically neutralized withcations outside of the polyhedral cage structure. In the polyhedral cagestructure, a central atom is disposed in the polyhedral cage structure,and the polyhedral cage structure includes a plurality of metal atoms,the same or different, bonded to oxygen atoms.

When at least one of the cations disposed outside of the polyhedral cagestructure is a proton, the compound is referred to as a heteropoly acid.When the cation is not a proton but an alkali metal ion or an ammoniumion, the compound is referred to as a polyoxometalate.

The heteropoly acid 30 and the polyoxometalate 30 have variousstructures, for example, Keggin structure, Dawson and Andersonstructure. These structures are determined according to geometricalstructure of predetermined heteropoly acid 30 compositions, and arechanged according to atomic radii of metal atoms present andcoordination chemistry. In an embodiment of the present invention, theheteropoly acid 30 and polyoxometalate 30 have the Keggin structure.

The Keggin-type heteropoly acid 30 may be one selected from the groupconsisting of H₄SiW₁₂O₄₀, H₄SiMo₁₂O₄₀, H₅SiVMo₁₁O₄₀, H₆SiV₂Mo₁₀O₄₀,H₇SiV₃Mo₉O₄₀, H₃PMo₁₂O₄₀, H₃PW₁₂O₄₀, (VO)_(1.5)PMo₁₂O₄₀,(VO)_(1.5)PW₁₂O₄₀, (TiO)_(1.5)PMo₁₂O₄₀, H(VO)PMo₁₂O₄₀, H(VO)PW₁₂O₄₀,H₆PV₃Mo₉O₄₀, H₅PV₂Mo₁₀O₄₀, H₅PV₂W₁₀O₄₀, H₆PV₃W₉O₄₀, H₄PV₂Mo₁₁O₄₀,H₄PVW₁₁O₄₀, RhPMo₁₂O₄₀, BiPMo₁₂O₄₀, HCrPVMo₁₁O₄₀, HBiPVMo₁₁O₄₀, and acombination thereof, but the present invention is not limited thereto.

In addition, the Keggin-type polyoxometalate 30 may be a salt formed bysubstituting protons in the heteropoly acid 30 with alkali metal ions orammonium ions. The alkali metal ions may be, for example, lithium ions,sodium ions, potassium ions, or cesium ions, but the embodiment of thepresent invention is not limited thereto.

The weight ratio of the phosphate platelets 10 to the Keggin-typepolyoxometalate 30 and/or the Keggin-type heteropoly acid 30 may be1000:1 through 1:1. When the amount of the Keggin-type polyoxometalate30 and/or the Keggin-type heteropoly acid 30 is smaller than the minimumof the range, ion conductivity is deteriorated. When the amount of theKeggin-type polyoxometalate 30 and/or the Keggin-type heteropoly acid 30is larger than the maximum of the range, the prevention of thepermeation of the liquid fuel is reduced.

The thickness of the ion-conductive inorganic membrane is 1 through 20μm. When the thickness of the ion-conductive inorganic membrane is lessthan 1 μm, the mechanical strength thereof decreases. When the thicknessof the ion-conductive inorganic membrane is greater than 20 μm, theelectric resistance thereof increases and the entire volume of the fuelcell increases.

Hereinafter, an ion-conductive organic/inorganic composite membraneaccording to an embodiment of the present invention will be described indetail.

The ion-conductive organic/inorganic composite membrane according to thecurrent embodiment of the present invention includes a porous basematerial and an ion-conductive inorganic material. The ion-conductiveinorganic material includes phosphate platelets 10; at least one siliconcompound 20 selected from the group consisting of silicate, siloxane,and silane; and Keggin-type polyoxometalate 30 and/or Keggin-typeheteropoly acid 30. The phosphate platelets 10 are three-dimensionallyconnected to each other by the at least one silicon compound 20. Theion-conductive inorganic material is coated on the surface of the porousbase material or impregnated throughout the entire porous base material.

The above-described ion-conductive inorganic membrane according to theprevious embodiment of the present invention does not include a basematerial and the ion-conductive inorganic material itself stands alone.The ion-conductive organic/inorganic composite membrane according to thecurrent embodiment of the present invention includes a porous basematerial and an ion-conductive inorganic material coated onto orimpregnated into the porous base material.

In the ion-conductive organic/inorganic composite membrane according tothe current embodiment of the present invention, the weight ratios ofthe phosphate platelets 10; the at least one silicon compound 20selected from the group consisting of silicate, siloxane, and silane;and the Keggin-type polyoxometalate 30 and/or the Keggin-type heteropolyacid 30 are the same as the weight ratios of the ion-conductiveinorganic membrane. In addition, the phosphate platelets 10; the atleast one silicon compound 20; and the Keggin-type polyoxometalate 30and/or the Keggin-type heteropoly acid 30 are the same as theion-conductive inorganic membrane described above.

The porous base material is not specifically limited to the examplesdescribed above, and any porous base material may be used as long as itis porous and has enough mechanical properties and thermal resistance tobe used as an electrolyte membrane. The porous base material may be apolymer resin, for example, polyvinyl alcohol, polyacrylonitrile, phenolresin, fluoropolymer-based resin, cellulose-based resin, or nylon resin.

Among them, ion-conductive materials can be used when coating anion-conductive inorganic material on the surface of the porous basematerial and when impregnating ion-conductive inorganic material intothe porous base material.

The polymer-based resin may be a polymer having a cation exchange group.The cation exchange group may be, for example, sulfonic acid group,carboxyl group, phosphoric acid group, imide group, sulfonimide group,sulfonamide group, or hydroxy group.

The polymer having a cation exchange group may be homopolymers orcopolymer of trifluoroethylene, tetrafluoroethylene, hexafluoropropylene(HFP), styrene-divinyl benzene, α,β,β-trifluorostyrene, styrene, imide,sulfone, phosphazene, etherether ketone, ethylene oxide,polyvinylidenefluoride (PVdF), polyphenylene sulfide, or aromatic group,and derivatives thereof. The polymer can be used in combination oralone.

The polymer having the cation exchange group may be a highly fluorinatedpolymer in which 90% of the total of fluorine atoms and hydrogen atomsbonded to carbon atoms in main chains and side chains of the polymer isfluorine atom.

The polymer having the cation exchange group includes sulfonate as thecation exchange group at one end of the side chain, and may be a highlyfluorinated polymer having the sulfonate group in which 90% of the totalof fluorine atoms and hydrogen atoms bonded to carbon atoms in mainchains and side chains of the polymer is fluorine atom.

The average diameter of pores formed in the porous base material may be0.05 through 10 μm. When the average diameter of the pores is less than0.05 μm, an inorganic material cannot be uniformly dispersed anddistributed in and on the porous base material. When the averagediameter of the pores is greater than 10 μm, the prevention of thecrossover of the liquid fuel is reduced.

The ion-conductive inorganic material may be coated on or impregnatedinto the porous base material. When the ion-conductive inorganicmaterial is impregnated, a thin ion-conductive inorganic material layeris formed on the surface of the porous base material.

The thickness of the ion-conductive inorganic material layer formed onthe porous base material may be 0.2 through 10 μm. When the thickness ofthe ion-conductive inorganic material layer is less than 0.2 μm, theprevention of the permeation of the liquid fuel is reduced. When thethickness of the ion-conductive inorganic material layer is greater than10 μm, the ion conductivity may decrease.

The thickness of the ion-conductive organic/inorganic composite membranemay be 50 through 200 μm. When the thickness of the ion-conductiveorganic/inorganic composite membrane is less than 50 μm, the mechanicalstrength thereof is insufficient. When the thickness of ion-conductiveorganic/inorganic composite membrane is greater than 200 μm, the entirevolume of the fuel cell increases and the ion conductivity thereofdecreases.

Hereinafter, a method of manufacturing the ion-conductive inorganicmembrane according to an embodiment of the present invention will bedescribed in detail.

100 parts by weight of phosphate platelet, 20 through 250 parts byweight of at least one silicon compound selected from the groupconsisting of silicate, siloxane, and silane, 0.1 through 10 parts byweight of Keggin-type polyoxometalate and/or Keggin-type heteropolyacid, and 10 through 30 parts by weight of an acid is mixed to form agel type mixture.

The acid acts as a catalyst in a reaction that converts the mixture intoa gel mixture, and may be sulfuric acid, nitric acid, hydrochloric acid,or phosphoric acid, and preferably nitric acid. The concentration of theacid may be 0.1 through 10.0 M. When the concentration of the acid isless than 0.1 M, the acid cannot properly act as a catalyst. Whenconcentration of the acid is greater than 10 M, a gel mixture cannot beproperly formed.

The mixing temperature for the gelation may be from 30 to 80° C. Whenthe mixing temperature is lower than 30° C., the reaction is so slowthat the reaction time is too long. When the mixing temperature ishigher than 80° C., the reaction rate is too fast that a desired shapemay not be obtained and phase separation may occur.

The gel mixture is applied onto a substrate and dried. The method ofuniformly applying the gel mixture onto a substrate may be a well-knownmethod to those of ordinary skill in the art, and thus the presentinvention is not limited thereto. Specifically, spin-coating,spray-coating, roll-coating, dip-coating, or knife-coating may beemployed.

The substrate may be formed of a material having a smooth surface, forexample, glass or quartz. To easily separate the ion-conductiveinorganic membrane and the substrate, a substrate on which a releaseagent is applied in advance may be used.

After the ion-conductive inorganic membrane is sufficiently dried andsolidified, the ion-conductive inorganic membrane may be peeled off fromthe surface of the substrate, thereby forming the ion-conductiveinorganic membrane.

Hereinafter, a method of manufacturing the ion-conductiveorganic/inorganic composite membrane according to an embodiment of thepresent invention will be described in detail.

A gel mixture is manufactured using the same method as in manufacturingthe ion-conductive inorganic membrane. The composition of the acidacting as a catalyst and conditions for forming the gel mixture are thesame as in the manufacture of the ion-conductive inorganic membrane.

The gel mixture may be coated onto or impregnated into a porous basematerial.

The method of uniformly applying the gel mixture onto the porous basematerial may be a well-known method to those of ordinary skill in theart, and thus the present invention is not limited thereto.Specifically, spin-coating, spray-coating, roll-coating, dip-coating, orknife-coating may be employed.

The method of impregnating the gel mixture into the porous base materialmay be a well-known method to those of ordinary skill in the art, andthus the present invention is not limited thereto. Specifically, theporous base material may be immersed in the gel mixture. The immersiontime may be 5 through 120 hours. When the immersion time is shorter than5 hours, the gel mixture is not sufficiently dispersed into the porousbase material so that prevention of the permeation of the liquid fuel isnot sufficient. When the immersion time is longer than 120 hours, theeffect of immersion is already saturated and it is not economical.

Hereinafter, a membrane electrode assembly (MEA) including theion-conductive inorganic membrane or the ion-conductiveorganic/inorganic composite membrane, according to an embodiment of thepresent invention will be described in detail.

The MEA according to an embodiment of the present invention includes acathode having a catalyst layer and a diffusion layer, an anode having acatalyst layer and a diffusion layer, and an electrolyte membranedisposed between the cathode and the anode, wherein the electrolytemembrane includes the ion-conductive inorganic membrane or theion-conductive organic/inorganic composite membrane of an embodiment ofthe present invention.

The cathode and anode including a catalyst layer and a diffusion layermay be well known to those of ordinary skill in the art. In addition,the electrolyte membrane includes the ion-conductive inorganic membraneor the ion-conductive organic/inorganic composite membrane according toan embodiment of the present invention. The ion-conductive inorganicmembrane or the ion-conductive organic/inorganic composite membraneaccording to the embodiments of the present invention may be solely usedas an electrolyte membrane, or may be combined with other ion-conductivemembranes.

Hereinafter, a fuel cell including the ion-conductive inorganic membraneor the ion-conductive organic/inorganic composite membrane of thepresent invention, according to an embodiment of the present inventionwill be described in detail.

The fuel cell according to an embodiment of the present inventionincludes a cathode having a catalyst layer and a diffusion layer, ananode having a catalyst layer and a diffusion layer, and an electrolytemembrane disposed between the cathode and the anode, wherein theelectrolyte membrane includes the ion-conductive inorganic membrane orthe ion-conductive organic/inorganic composite membrane according to anembodiment of the present invention.

The cathode and anode including the catalyst layer and the diffusionlayer may be well known to those of ordinary skill in the art of thefuel cell. The electrolyte membrane includes the ion-conductiveinorganic membrane or the ion-conductive organic/inorganic compositemembrane according to an embodiment of the present invention. Theion-conductive inorganic membrane or ion-conductive organic/inorganiccomposite membrane according to an embodiment of the present inventionmay be used as an electrolyte membrane alone or in combination withother ion-conductive membranes.

The fuel cell according to an embodiment of the present invention can bemanufactured using conventional methods, and thus detailed descriptionsthereof are omitted.

An embodiment of the present invention will be explained in detail withreference to the following examples. The following examples are forillustrative purposes and are not intended to limit the scope of theinvention.

Examples 1 Through 3

5 g of exfoliated phosphate platelet, 5 g of TEOS, and 0.05 g ofNa₃PW₁₂O₄₀.xH₂O as a polyoxometalate were mixed, and then 0.5 ml of 1.0M nitric acid was added thereto, thereby forming a gel mixture at 50° C.

Mixed cellulose ester (see FIG. 2A) was used as a porous base material.The pore size of the porous base material was 3 μm, and the thicknessthereof was 110 μm.

The gel mixture was repeatedly spin-coated on the porous base materialso that coating layers respectively having thicknesses of 1, 3, 5 μmwere formed, thereby forming an ion-conductive organic/inorganiccomposite membrane. The spin-coating was performed at 1000 RPM for 10seconds.

FIG. 3 is a scanning electron spectroscopy (SEM) image of a surface ofan ion-conductive organic/inorganic composite membrane of Example 2.

Examples 4 Through 6

5 g of exfoliated phosphate platelet, 5 g of TEOS, and 0.05 g ofNa₃PW₁₂O₄₀.xH₂O as a polyoxometalate were mixed, and then 0.5 ml of 1.0M nitric acid was added thereto, thereby forming a gel mixture at 50° C.

Cellulose nitrate (see FIG. 2B) was used as a porous base material. Thepore size of the porous base material was 8 μm, and the thicknessthereof was 120 μm.

The porous base material was immersed in the gel mixture for 36 hours sothat the gel mixture was impregnated thereinto. The immersion processwas repeated to form coating layers respectively having thicknesses of1, 3, 5 μm, thereby forming an ion-conductive organic/inorganiccomposite membrane.

FIGS. 4A and 4B are SEM images illustrating an inner cross-section and asurface of the ion-conductive organic/inorganic composite membranemanufactured in Example 5, respectively.

FIG. 5 is a graph of ion conductivity measured at 90° C. of theion-conductive organic/inorganic composite membranes manufactured inExamples 1 through 6. Referring to FIG. 5, the ion conductivities of theion-conductive membranes according to Examples 1 through 6 are notsignificantly deteriorated compared with a conventional polymerelectrolyte membrane. In addition, the ion-conductive inorganic materialimpregnated membranes of Examples 4 through 6 generally have higher ionconductivity than the surface-coated composite membranes of Examples 1through 3.

When the thickness of the coated surface of the ion-conductive inorganicmaterial is 3 μm, the composite membrane has high ion conductivity.

Comparative Example 1

A chamber was separated using a NAFION membrane having a thickness of150 μm and sealed to prevent the leaking of a liquid. 20 ml of deionizedwater was filled in one side of the chamber separated by the NAFIONmembrane, and 20 ml of a 5.0 M methanol solution was filled in the otherside of the chamber. The concentration of the methanol solution wasmeasured with time. The results are shown in Table 1.

Examples 7 and 8

Concentrations of the methanol solution were measured in the same manneras in Comparative Example 1 except ion-conductive organic/inorganiccomposite membranes of Example 2 and 5 were used instead of the NAFIONmembrane, respectively. The results are shown in Table 1.

TABLE 1 Time (minutes) Example 7 Examples 8 Comparative Example 1 0 5.05.0 5.0 10 5.0 5.0 4.5 20 4.9 4.9 4.0 30 4.9 4.9 3.6 40 4.9 4.9 3.2 504.8 4.9 2.9 60 4.8 4.9 2.6

Referring to Table 1, the ion exchange organic/inorganic compositemembrane of an embodiment of the present invention prevents thecrossover of methanol. However, for the conventional NAFION membrane,the methanol concentrations in both sides of the membrane become aboutequal after 60 minutes due to the methanol crossover.

Example 9

5 g of exfoliated phosphate platelet, 5 g of1,2-bis(trimethoxysilyl)ethane, and 0.05 g of Na3PW12O40.xH2O as apolyoxometalate were mixed, and then 0.5 ml of 1.0 M nitric acid wasadded thereto, thereby forming a gel mixture.

The gel mixture was applied onto a quartz substrate and dried for 24hours, thereby forming a membrane. The membrane was peeled off from thequartz substrate to form an ion-conductive inorganic membrane, thusforming the membrane.

Example 10

5 g of exfoliated phosphate platelet, 5 g of octyltrimethoxysilane, and0.05 g of Na3PW12O40.xH2O as a polyoxometalate were mixed, and then 0.5ml of 1.0 M nitric acid was added thereto, thereby forming a gelmixture.

The gel mixture was applied onto a quartz substrate and dried for 24hours, thereby forming a membrane. The membrane was exfoliated from thequartz substrate to form an ion-conductive inorganic membrane, thusforming the membrane.

The electrolyte membrane having an ion-conductive inorganic membrane oran ion-conductive organic/inorganic composite membrane effectivelyprevents crossover of the liquid fuel without the reduction of ionconductivity in a liquid fuel cell, thereby allowing the production offuel cells having excellent performance.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1. An ion-conductive inorganic membrane comprising: phosphate platelets;at least one silicon compound selected from the group consisting ofsilicate, siloxane, and silane; and a Keggin type polyoxometalate and/ora Keggin-type heteropoly acid, wherein: the phosphate platelets arethree-dimensionally connected to each other via the at least one siliconcompound, the Keggin-type heteropoly acid is an acid selected from thegroup consisting of H₄SiW₁₂O₄₀, H₄SiMo₁₂O₄₀, H₅SiVMo₁₁O₄₀,H₆SiV₂Mo₁₀O₄₀, H₇SiV₃Mo₉O₄₀, H₃PMo₁₂O₄₀, H₃PW₁₂O₄₀, (VO)_(1.5)PMo₁₂O₄₀,(VO)_(1.5)PW₁₂O₄₀, (TiO)_(1.5)PMo₁₂O₄₀, H(VO)PMo₁₂O₄₀, H(VO)PW₁₂O₄₀,H₆PV₃Mo₉O₄₀, H₅PV₂Mo₁₀O₄₀, H₅PV₂W₁₀O₄₀, H₆PV₃W₉O₄₀, H₄PV₂Mo₁₁O₄₀,H₄PVW₁₁O₄₀, RhPMo₁₂O₄₀, BiPMo₁₂O₄₀, HCrPVMo₁₁O₄₀, HBiPVMo₁₁O₄₀, and acombination thereof, and the Keggin-type polyoxometalate is a saltformed by substituting protons of the Keggin-type heteropoly acid withone of alkali metal ions and ammonium ions.
 2. The ion-conductiveinorganic membrane of claim 1, wherein a weight ratio of the phosphateplatelets to the at least one silicon compound selected from the groupconsisting of silicate, siloxane, and silane is 1:0.2 through 1:2.5. 3.The ion-conductive inorganic membrane of claim 1, wherein a weight ratioof the phosphate platelets to the Keggin-type polyoxometalate and/or tothe Keggin-type heteropoly acid is 1000:1 through 1:1.
 4. Theion-conductive inorganic membrane of claim 1, wherein the phosphateplatelets are α-zirconium phosphate.
 5. The ion-conductive inorganicmembrane of claim 1, wherein the silicon compound is selected from thegroup consisting of colloidal silica, polydimethylsiloxane (PDMS),alkoxysilane, chlorosilane, and a combination thereof.
 6. Theion-conductive inorganic membrane of claim 1, wherein the siliconcompound is selected from the group consisting oftetraethylorthosilicate (TEOS), tetramethylorthosilicate (TMOS),1,2-bis(trimethoxysilyl)ethane, octyltrimethoxysilane, tetraethylsilicate, tetraisopropyl silicate, tetramethyl silicate,tetramethoxysilane, tetraethoxysilane, tetraisopropoxysilane,propyltrimethoxysilane, ethyltrimethoxysilane, methyltrimethoxysilane,methyltriethoxysilane, aminoalkoxysilane, silicontetrachloride (SiCl₄),trichloro(dichloromethyl)silane (Cl₂CHSiCl₃), trichloro(hexyl)silane(CH₃(CH₂)₅SiCl₃), trichloro(isobutyl)silane ((CH₃)₂CHCH₂SiCl₃),trichloro(octadecyl)silane (CH₃(CH₂)₁₇SiCl₃), trichlorooctylsilane,trichloro(phenethyl)silane (C₆H₅CH₂CH₂SiCl₃), trichloro(phenyl)silane,trichloro(propyl)silane, trichloro(methyl)silane,trichloro(chloromethyl)silane, dichloro(dimethyl)silane,isobutyl(trimethoxy)silane, and a combination thereof.
 7. Theion-conductive inorganic membrane of claim 1, wherein the thickness ofthe phosphate platelets ranges from 0.5 to 10 nm.
 8. The ion-conductiveinorganic membrane of claim 1 having a thickness of 1 through 20 μm. 9.A membrane electrode assembly (MEA) comprising a cathode having acatalyst layer and a diffusion layer, an anode having a catalyst layerand a diffusion layer, and an electrolyte membrane disposed between thecathode and the anode, wherein the electrolyte membrane comprises theion-conductive inorganic membrane according to claim
 1. 10. A fuel cellcomprising a cathode having a catalyst layer and a diffusion layer, ananode having a catalyst layer and a diffusion layer, and an electrolytemembrane disposed between the cathode and the anode, wherein theelectrolyte membrane comprises the ion-conductive inorganic membraneaccording to claim 1.